The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
Staphylococcus aureus (S. aureus) is a cause of a variety of conditions in humans, including skin infections (e.g. folliculitis, styes, cellulitis, impetigo, and furunculosis), pneumonia, mastitis, phlebitis, meningitis, scalded skin syndrome, osteomyelitis, urinary tract infections, and food poisoning. In addition, the Centers for Disease Control and Prevention (CDC) has recognized methicillin-resistant S. aureus (MRSA) as a growing problem in the healthcare setting as it is one of the major causes of hospital acquired infections such as hospital-acquired (HA or nosocomial) infection of surgical wounds.
MRSA is one of the two most rampant antibiotic resistant pathogens; vancomycin-resistant enterococcus is the other (Society for Healthcare and Epidemiology, SHEA guidelines 2003). Over 50% of nosocomial infections in intensive care units are due to MRSA (National Nosocomial Infections Surveillance System, NNIS report, January 1992-June 2004). Accordingly, MRSA represents a significant threat to public health.
Methicillin resistance is caused by the acquisition of an exogenous gene mecA that encodes penicillin-binding protein (PBP2a or PBP2′), which exhibits a low affinity for β-lactam antibiotics (Wielders and Fluit, 2002). mecA is carried on a mobile genetic element called Staphylococcal cassette chromosome mec (SCCmec) which also contains the ccr gene complex encoding the recombinases necessary for the element's mobility. The SCCmec cassette is a large element that can move in and out of the S. aureus genome.
The mecA gene also is found in coagulase-negative Staphylococcus (CNS) strains that are less pathogenic than S. aureus. These strains include S. epidermidis, S. haemolyticus, S. saprophyticus, S. capitis, S. warneri, S. sciuri and S. caprae. Some of these other strains of Staphylococcus inhabit the same environments as S. aureus such as the anterior nares and the skin. It follows that clinical samples such as nasal swabs or wound swabs could potentially contain a mixture of more than one Staphylococcal species. Therefore, detection of mecA alone is not sufficient to identify MRSA directly from clinical sample. Because identification of MRSA is of greater clinical significance than the other Staphylococcus species due to its increased pathogenicity and toxicity, it is desirable to have a diagnostic assay that distinguishes MRSA from the other staphylococcal strains containing the mecA gene.
More recently, an additional mec gene, named mecC, was discovered which also confers beta-lactam resistance. mecC (GenBank accession no. FR821779), formerly referred to as mecA homologue in early publications, is present on a SCCmec XI element (GenBank accession no. FR823292). The mecC gene has been described in human and bovine S. aureus and it encodes a protein that has <63% aa identity with PBP2a encoded by mecA.
Hospital acquired (HA) MRSA is typically controlled by monitoring patients and personnel for infection. Contact precautions and/or patient isolation may be appropriate when an infection develops or to prevent infections to individuals particularly at risk. The prevalence of community acquired (CA) MRSA is also increasing. CA-MRSA is defined as MRSA acquired in persons with no known risk factors for MRSA infection (e.g. recent hospitalization, contact with infected patient). Because a quick and reliable identification of MRSA has become important for the diagnosis and treatment of infected patients, as well as for implementation and management of hospital infection control procedures, it is desirable to have a diagnostic assay that detects the presence of S. aureus and, in particular, the presence of MRSA. It is additionally desirable to have such an assay that does not require a front-end specimen preparation process separate from the detection system.